1. Field of the Invention
The present invention generally relates to a switching power supply, and more particularly, to a switching power supply which can be provided with a wide range of input voltage and can produce a large amount of load current by a supplementary power-supply circuit.
2. Description of the Related Art
At present, a variety of switching power supplies are proposed. A typical switching power supply comprises a transformer having primary, secondary, and tertiary windings, a supplementary power-supply circuit connected to the tertiary winding, and a control circuit operating by output power from the supplementary power-supply circuit. In the switching power supply, the control circuit controls a switching element connected with the primary winding to turn on and off, and power is supplied to a load connected with the secondary winding.
In the following, a description will be given of one example of a prior-art switching power supply, by referring to FIG. 1. FIG. 1 shows a schematic diagram of the one example of the prior art switching power supply.
In the prior-art switching power supply shown in FIG. 1, an alternating-current power supply 1 is connected to a primary-side rectifying-and-smoothing circuit 3 having a diode bridge 2 and an electrolytic capacitor C1. An output side of the primary-side rectifying-and-smoothing circuit 3 is connected to a primary winding N1 of a transformer 5 and to a switching element Q1 having, an FET through a snubber circuit 4 having serial and parallel circuits constructed with a capacitor C2, a resistor R1, and a diode D1. A secondary winding N2 of the transformer 5 is connected with an output terminal 7 of the switching power supply through a secondary-side rectifying-and-smoothing circuit 6 having a diode D2 and an electrolytic capacitor C3, the output terminal 7 being connected to a load device 8.
To a tertiary winding N3 of the transformer 5, a supplementary power-supply circuit 10 having a diode 3, a resistor R2, and an electrolytic capacitor C4 is connected. To the supplementary power-supply circuit 10, a pulse-width modulation (PWM) control circuit 11 controlling on-and-off of the switching element Q1 is connected. And also, a start-up circuit 12 whose input side is connected with the diode bridge 2 is connected to the supplementary power-supply circuit 10 for providing current thereto. The PWM control circuit 11 is connected to the switching element Q1 through a gate circuit 13, and is connected to the output terminal 7 through a feedback circuit 14. The feedback circuit 14 detects an output voltage of the output terminal 7, and feeds it back to the PWM control circuit 11. The feedback circuit 14 includes a photocoupler 15 whose photo-detection part and photo-radiation part are respectively connected to the PWM control circuit 11 and the output terminal 7, a Zener diode D4 constituting a shunt regulator, a capacitor C5, and resistors R3, R4, R5.
Next, an operation of the prior-art switching power supply will be discussed. In FIG. 1, when the alternating-current power supply 1 is turned on to supply the power, the alternating-current power is rectified to be a direct-current voltage in the primary-side rectifying-and-smoothing circuit 3. The direct-current voltage is applied to the transformer 5 and the switching element Q1. The direct-current voltage is also applied to the supplementary power-supply circuit 10 through the start-up circuit 12, and passes the resistor R2 and the diode D3 of the supplementary power-supply circuit 10. In the supplementary power-supply circuit 10, the electrolytic capacitor C4 is charged by a pulsating start-up current from the start-up circuit 12. At this time, since an electric charge stored in the electrolytic capacitor C4 is prevented from discharging by the diode D3, a voltage across the electrolytic capacitor C4 increases. When the increased voltage of the supplementary power-supply circuit 10 reaches a start-up voltage of the PWM control circuit 11, the PWM control circuit 11 starts to operate.
An output signal of the PWM control circuit 11 controls an on-and-off operation of the switching element Q1 through the gate circuit 13. When the switching element Q1 turns on, a current flows through the primary winding N1 of the transformer 5, and energy is stored in the transformer 5. When the switching element Q1 turns off, the stored energy in the transformer 5 is transferred to the secondary winding N2 of the transformer 5, and is rectified to a direct-current voltage in the secondary-side rectifying-and-smoothing circuit 6. The direct-current voltage supplies power to the load device 8 from the output terminal 7. An output voltage of the output terminal 7 is detected in the feedback circuit 14, and is fed back to the PWM control circuit 11. By the PWM control circuit 11, the output voltage of the output terminal 7 is stably controlled to be a given voltage. Further, polarization of the tertiary winding N3 of the transformer 5 is set so that when the switching element Q1 turns on, the diode D3 connected in series to the tertiary winding N3 turns on to charge the electrolytic capacitor C4. And, a supplementary power-supply voltage Vcc substantially proportional to the input voltage is obtained.
In the prior-art supplementary power-supply circuit, when the PWM control circuit 11 is formed by an integrated circuit (IC), a rated voltage of the IC and a voltage between a gate and a drain of the switching element Q1 respectively have limits. In this case, the supplementary power-supply voltage Vcc needs to have a selection range of approximately 10 to 24 V. When an input voltage range of the power supply is relatively large, a varying range of the supplementary power-supply voltage may be over the selection range of 10 to 24 V. Therefore, an additional voltage regulator, for example, is necessary. Accordingly, the number of components increases, and, thus, cost of the switching power supply also increases. Further, when the input voltage is relatively large, the supplementary power-supply voltage is also large in proportion to the input voltage. Therefore, consumption power of the PWM control circuit 11 connected to the supplementary power-supply circuit 10 also increases. As a result, there is thus a problem in that overall transforming efficiency of the switching power supply is degraded.
To resolve the above problem, another method is proposed. In the method, in the switching power supply shown in FIG. 1, only the polarization of the tertiary winding N3 of the transformer 5 is reversed. In this case, when the switching element Q1 turns on, energy is stored in the transformer 5. And, when the switching element Q1 turns off, the diode D3 connected in series to the tertiary winding N3 turns on to charge the electrolytic capacitor C4. The supplementary power-supply voltage Vcc substantially proportional to the secondary-side output voltage is obtained.
In this method, even if the input voltage range of the power supply is relatively large, a varying range of the supplementary power-supply voltage may be reduced, and the IC for the PWM control circuit 11 may stably operate. However, even if the input voltage of the switching power supply is extremely small, the switching power supply continues to operate. At this time, the current flowing through the switching element Q1 increases in inverse proportion to the input voltage. Therefore, a power loss in the switching element Q1 increases and, as a result, the switching element Q1 is heated. When the PWM control circuit 11 cannot track the variation of the input voltage, unstable output voltage is produced to the output terminal 7. In this case, the unstable output voltage may cause error operations in fabricated devices and, in particular, the IC for the PWM control circuit 11.
Furthermore, in the prior-art supplementary power-supply circuit 10, the start-up circuit 12 for generating the start-up current of the supplementary power-supply circuit 10 is connected to a terminal outputting the direct-current voltage of the primary-side rectifying-and-smoothing circuit 3. Therefore, when trouble occurs in the switching power supply or the load device 8 and a protecting operation mode is set, even if the alternating-current power supply 1 is turned off, the current continues to flow through the PWM control circuit 11 because the electric charge remains in the primary-side rectifying-and-smoothing circuit 3. In this way, since the protecting operation mode is set, it takes a few minutes until the PWM control circuit 11 is reset. As a result, it takes a long time for an operation test, for example, of the switching power supply. The above problem may be resolved by an additional circuit forcibly resetting the PWM control circuit 11 when the alternating-current power supply 1 is turned off. However, by the additional circuit, the number of components increases, and the cost of the switching power supply may increase.